1. Field of the Invention
The present invention relates to a phase retarder film for improving viewing angle characteristics of liquid crystal displays.
2. Description of Related Art
The liquid crystal display (hereinafter, referred to as LCD) most widely used at present is a twisted nematic LCD (hereinafter, referred to as TN-LCD) of normally white (hereinafter, referred to as NW) mode in which a liquid crystal cell such that nematic liquid crystal is held between a pair of glass substrates having transparent electrodes is sandwiched between a pair of linearly polarizing films disposed so that the absorption axes thereof are perpendicular to each other. The nematic liquid crystal has an orientation structure such that the helical axis is normal to the glass substrates and the twist angle thereof is approximately 90 degrees.
When no voltage is applied, the NW mode TN-LCD is in white state since the incident linearly polarized light exits after being rotated 90 degrees because of the octical rotatory power of the liquid crystal cell. When a voltage is applied, the NW mode TN-LCD is in black state since the liquid crystal molecules stand up with respect to the glass substrates to cause the optical rotatory power to disappear and the incident linearly polarized light exits as it is. Gray scale is performed by using the white state, the black state and intermediate states therebetween.
However, the nematic liquid crystal used for the LCD has a rod like molecular structure and exhibits positive refractive index anisotropy where the refractive index is higher in the direction of the molecular axis than in the normal direction to the molecular axis, so that the polarization condition of light: obliquely passing through the liquid crystal cell varies in a different direction from the direction of the normal to the liquid crystal cell because of a phase difference due to the refractive index anisotropy of the liquid crystal. For this reason, when the display on the LCD is viewed from angles other than the normal direction to the LCD, viewing angle characteristics are shown such that the contrast decreases and that gray scale inversion takes place.
Since the viewing angle characteristics are caused by the refractive index anisotropy of the liquid crystal molecules, an improvement being examined using a phase retarder film exhibiting a refractive index anisotropy opposite to that of the liquid crystal to compensate the phase difference due to the refractive index anisotropy of the liquid crystal molecules.
In improving the viewing angle characteristics, significant effects are obtained by improving the viewing angle characteristics in black state, namely in voltage applied state. In voltage applied state, liquid crystal molecules are oriented in a direction nearly perpendicular to the glass substrates. Regarding the liquid crystal cell in this state as a positive refractive index anisotropic substance having its optic axis in the normal direction to the glass substrates, Japanese Laid-open Patent Publications No. 2-015239 and 3-103823 disclose to use a phase retarder film having its optic axis in the normal direction to the film and having negative refractive index anisotropy as a phase retarder film for compensating the positive refractive index anisotropy of the liquid crystal cell.
Moreover, in the actual LCDs, liquid crystal molecules in the vicinity of the glass substrates remain tilted even in voltage applied state because of the restraint force of the orientation lavers on the substrates. In view of this fact, Japanese Laid-open Patent Publications No. 63-239491 and 6-214116 describe to use a phase retarder film having its optic axis tilted from the normal direction to the film and having negative refractive index anisotropy.
Further, Japanese Laid-open Patent Publications No. 5-080323 and 7-306406 and WO96/10773 describe that the viewing angle characteristics can be improved by using a phase retarder plate having its optic axis tilted from the normal direction to the film although having the same positive refractive index anisotropy as the liquid crystal has.
Recently, the TN-LCD has seen put to various uses such as personal computers, so that it is strongly demanded that the TN-LCD have greater size, higher definition, wider viewing angle, lighter weight and lower cost.
The use of the phase retarder film having its optic axis in the normal direction to the film and having negative refractive index anisotropy does not always show sufficient effect of the viewing angle characteristic improvement.
In the case of the use of the phase retarder film having its optic axis tilted from the normal direction to the film and having negative refractive index anisotropy, a satisfactory effect of the viewing angle characteristics improvement is obtained. However, according to a method in which the phase retarder film is cut from a polymer block oriented to exhibit negative refractive index anisotropy so that its optic axis is tilted from the normal direction to the film as disclosed in Japanese Laid-open Patent Publication No. 63-239421, it is difficult to efficiently obtain an uniform and large size phrase retarder film, and according to a method using a discotic liquid crystal compound as disclosed in Japanese Laid-open Patent Publication No. 6-214116, it is difficult to produce the liquid crystal compound and to uniformly orient the liquid crystal. In addition, this method is disadvantageous in cost.
In the case of the use of a chase retarder film having its optic axis tilted from the normal direction to the film and having positive refractive index anisotropy, although a satisfactory effect of the viewing angle characteristics improvement is obtained, according to a method in which the phase retarder film is cut from a polymer block oriented to have positive refractive index anisotropy so that its optic axis is tilted from the normal direction to the film as disclosed in Japanese Laid-open Patent Publication No. 5-080323, it is difficult to efficiently obtain an uniform and large size phase retarder film. Moreover, according to a method in which the phase retarder plate is formed by obliquely evaporating an inorganic dielectric onto the glass substrates as described in Applied Optics Vol. 28 (1989), pp. 2466-2482 disclosed in U.S. Pat. No. 5,504,603 and a method in which the phase retarder plate is formed by obliquely evaporating Ta.sub.2 O.sub.5 onto the glass substrates as disclosed in WO96/10773, although a phase retarder plate can be obtained having its optic axis tilted from the normal direction to the plate and having positive refractive index anisotropy, since these methods use glass substrates, only a phase retarder plate can be obtained that is heavy in weight, lacks flexibility and is inferior in mass productivity.
Thus, for the phase retarder film used for improving viewing angle characteristics of the TN-LCD, no film has been found that meets the demands of the market including not only improving the viewing angle characteristics but also other performance.
In view of these circumstances, the inventors have developed a phase retarder film having a large area, being lightweight and being excellent in mass productivity in which the principal optic axis is tilted from the normal direction to the film by providing an oblique evaporation layer of an inorganic dielectric on at least one surface of a transparent polymer film with an intermediate layer of acrylic resin and the like between, so that the present invention has completed.